This refers to the cervix in a woman's vagina. The cervix is located at the back of the vagina, hence the "bottom of the pussy hole". Men with a long enough penis can bump into the cervix during sex.

Generally, hitting the bottom of the pussy hole is painful for women, while others find it that it feels good. Most women report that stimulation of the cervix during intercourse is very painful and results in a sensation similar to being kicked in the stomach.

Jennifer told all of her girlfriends she wanted to fuck a nigga with a big dick during spring break. She found her mandingo in Cabo. Back in her hotel room he thrust his meat into her like a madman! Quickly she went from yelling YES to OMG OUCH as soon as he hit the bottom of the pussy hole.

Lesson of this story:

What is the difference between "Oooh!" and "Aaah!"?
About three inches.